(1) Field of the Invention
The present invention relates to a developer and a developing process. More particularly, the present invention relates to a two-component developer comprising toner particles and carrier particles supporting the toner particles, and a developing process using this developer. Furthermore, the present invention relates to a developer and developing process characterized in that even by a developing mechanism in which the voltage difference between a photosensitive material and a developing sleeve (developer-supporting member) is small, an image having a high density is obtained at a high resolution and so-called carrier dragging is not caused. Still further, the present invention relates to a developer and developing process characterized in that even if a toner having a low electroconductivity is used, an image having a high density is obtained at a high resolution and so-called carrier dragging is not caused.
(2) Description of the Related Art
In the field of commercial electrophotography, magnetic brush development using a two-component magnetic developer is widely adopted for developing an electrostatic image. As the two-component type magnetic developer, there is widely used a mixture comprising a magnetic carrier composed of an iron powder or sintered ferrite particles and a toner composed of particles formed by dispersing additives such as a colorant and a charge-controlling agent in a binder resin.
An ordinary developing mechanism in which a developer as described above is used has a structure as shown in FIG. 1. More specifically, a box-shaped toner supply mechanism 4 is arranged on the developing mechanism 2 and a toner is supplied from above. The toner 6 is fed into a developing device 10 disposed below through a supply opening 8 equipped with a feeder and is stirred together with a carrier in the developing device 10 by stirrers 12 to form a two-component type developer 14.
A developing sleeve (developer-supporting member) 16 equipped with many magnetic poles is arranged in the developing device 10. The developer 14 having the frictionally charged toner is supplied into the developing sleeve and a magnetic brush 18 of the developer is formed on the surface of the sleeve by a magnetic force. The length of the magnetic brush 18 is adjusted by a brush-cutting mechanism (doctor blade) 20, and a uniform layer of the developer is formed on the surface of the developing sleeve 16. This developer layer is delivered to the nip position to a surface photosensitive layer 24 of an electrophotographic photosensitive material drum (image carrier) 22. The photosensitive material drum 22 is arranged apart by a distance DD-S from the developing sleeve 16, and the developing sleeve 16 and photosensitive material 22 are rotatably supported and are driven so that the moving directions of the sleeve 16 and drum 22 are the same at the nip position (the rotation directions are reverse to each other).
A corona charger 26 connected to a variable high voltage power source 25 and an optical system 28 for the light exposure are arranged around the photosensitive material drum 22 upstream of the developing device 10 to form an electrostatic latent image having a predetermined surface voltage. A bias power source 33 equipped with a voltage-adjusting mechanism 30 is connected between the photosensitive drum 22 and the developing sleeve 12 so that an optional value voltage (bias voltage) which has the same polarity as that of the surface voltage and is lower than the surface voltage is applied onto the photosensitive layer 24. A transfer mechanism 34 for transferring a toner image to a copying paper is arranged around the photosensitive layer 24 downstream of the developing zone.
In the above-mentioned structure, the developer 14 forms the magnetic brush 18 on the developing sleeve 16 and at the nip position, this magnetic brush 18 reacts with the electrostatic latent image of the photosensitive layer 24 to form a visible image of the toner on the photosensitive layer 24.
At this image-forming step, it is required that the optical density of the image area should be high and the adhesion of the toner to the background (so-called fogging) should be small. The reason is that this fogging includes a risk of the transfer of the lowly charged toner to the background. As the means for obtaining an image having reduced fogging, there can be considered a method of increasing the bias voltage. If the bias voltage is increased, the quantity of light necessary for the photosensitive layer can be reduced, and therefore, the development speed can be increased.
However, the increase of the bias voltage results in enhancement of the charge repulsion between the magnetic carrier and the developing sleeve, and therefore, the phenomenon of so-called carrier dragging, that is, the phenomenon that the carrier is transferred together with the toner to the photosensitive material, is often caused.
As regards the developing conditions adopted in the conventional Se type photosensitive material drum for the development, the distance DD-S between the photosensitive drum and the developing sleeve and the cutting length of the magnetic brush (the distance between the developing sleeve and the doctor blade) are adjusted to more than about 1 mm, and the development voltage difference is set at such a high level as 550 to 600 V. Under these conditions, troubles such as carrier dragging are not caused even if a conventional developer is used.
Recently, diminishment of the DD-S width is tried for improving the image density. However, if the DD-S width is diminished, the above-mentioned carrier dragging is caused.
An organic photosensitive material which has a good processability and is advantageous in the manufacturing cost and has a large freedom of the design of functions is recently used as the photosensitive material for the electrophotography. The organic photosensitive material includes a negatively chargeable type and a positively chargeable type. Since the negatively chargeable type often induces contamination of the copying environment, use of the positively chargeable organic photosensitive material is now expected.
In this positively chargeable photosensitive material, however, the residual voltage is apt to become larger than in the conventional Se type photosensitive material, and therefore, in the case where the positively chargeable photosensitive material is used, the bias voltage should be maintained at a level higher than 250 V. As shown in FIG. 2, if the bias voltage is high, it is obvious that the carrier loss is frequently caused in case of the conventional developer (broken line .DELTA.). Recently, under the necessity of increasing the bias voltage of the photosensitive material, it is required that the development voltage difference, that is, the difference between the surface voltage and the bias voltage should be up to 500 V. In case of the conventional developer and developing process, however, if the development voltage difference is controlled to 500 V or less, reducti on of the image density is caused and no satisfactory results can be obtained.
When a color image is formed, a dye or pigment of a desired color, other than a black dye such as carbon, is selected, and in this case, the kinds of toners that can be selected are limited and the electric resistance (the reciprocal of the electroconductivity) of the color toner tends to increase. Therefore, it sometimes happens that the image density (ID) cannot be maintained at a high level. According to the conventional technique, in order to solve these problems, the particle size of the magnetic carrier is reduced and the feed quantity of the toner is increased, or the electric resistance of the magnetic carrier is reduced.
However, even if the physical properties of the magnetic carrier are thus changed, a sufficient image density cannot be given to the color image, and especially in case of a red toner having a low electroconductivity, it often happens that no satisfactory image density can be obtained. Furthermore, there can be considered a method in which the DD-S width is reduced below the DD-S width in the conventional technique to increase the density of the color image. In this method, however, a stress is imposed and carrier dragging is caused.